Owing to their unique properties, such as water repellency, surface activity, thermal stability, etc., siloxanes are used in numerous industrial applications. These include the stabilization of polyurethane foams. For the industrial production of polyurethane foams, polyoxyalkylene-polysiloxane block copolymers are usually used as stabilizers. They emulsify the raw materials used, stabilize the foam during the preparation process and permit the formation of a uniform pore structure having the desired cell fineness and open-cell character. By means of suitable silicone surfactants tailored to the respective raw materials and production processes, it is thus possible to optimize foam properties and preparation processes.
The usability of these polyoxyalkylene-polysiloxane block copolymers in polyurethane foam applications is determined by the equilibrium to be established for the polysiloxane blocks and the polyoxyalkylene blocks. Thus, the composition as well as the distribution of these two blocks are of key importance. The possibilities of varying the polyoxyalkylene blocks are described, inter alia, in EP-A-0 867 464. The polyoxyalkylene blocks may be linked to the polysiloxane via an SiC bond which is stable to hydrolysis or an Si—OC bond which has limited stability to hydrolysis. The polysiloxane may be modified with regard to the number of siloxane units, the branches and the number of possible linkages with the polyoxyalkylene.
A standard method for the preparation of SiC-linked polyoxyalkylene-polysiloxane block copolymers is hydrosilylation. Here, an SiH-containing siloxane is reacted with organic groups which contain a terminal double bond. If the SiH-containing siloxanes are prepared by standard processes (equilibration), the SiH functionalities are randomly distributed along the siloxane chain. In the case of a predetermined functionalization density which ensures the required compatibility of the siloxane with the polyurethane raw materials, the average length of the unmodified regions in the siloxane chains is determined as a quotient of the total chain length and number of SiH functionalities, i.e. the achievable length of unmodified siloxane is limited by the predetermined functionalization density. If the desired functionalization density is high, only short regions of unmodified siloxane can be obtained in the chains, resulting in a deterioration of the actual properties of the siloxane. If a low functionalization density is established, the proportion of unmodified or extremely weakly modified siloxanes increases owing to the random distribution, particularly in the case of short chain lengths. In the preparation of polyurethane foams, these unmodified siloxanes have an undesired antifoam and destabilizing effect because of a lack of sufficient compatibility with the raw materials.
One possibility for optimizing the polyoxyalkylene-polysiloxane block copolymers as a polyurethane foam stabilizer consists in the partial elimination of the random distribution of the functionalization by siloxanes having a block structure which permits decoupling of functionalization density and maximum length of the unmodified regions and avoids fractions of undesired unfunctionalized siloxanes. With the aid of this technique, it is possible in principle to combine the strong surface activity of “pure” siloxane and the organic modification without disadvantages by being able to realize with the formulation compatible siloxanes with simultaneous presence of long unmodified siloxane chains. In this way, stabilizers which greatly reduce the surface tension and have sufficient polymer compatibility can be formulated.
There have been in the past some efforts to synthesize siloxanes which have a block structure and are composed of blocks of unmodified siloxane and blocks of organomodified siloxane.
Thus, U.S. Pat. No. 5,475,076 describes compounds which contain blocks of unmodified and modified siloxane. However, these compounds always contain at least one crosslinkable group (T unit) via which a network is built up between the individual siloxane chains. As a result of this crosslinking, siloxane chains are no longer present; rather, a sort of thermosetting plastic forms.
In contrast, DE-A-31 26 343 describes a process in which α,ω-hydroxy-functional siloxanes which are composed of monoorganohydrogensiloxane units and optionally diorganosiloxane units are reacted with α,ω-hydroxy-functional siloxanes which are composed exclusively of diorganosiloxane units. However, the reaction is effected under acidic conditions so that not only is the condensation of the hydroxyl groups catalyzed but at the same time an equilibration also takes place, resulting in a redistribution of the SiH functionalities along the chain. Thus, a block structure is not realized; rather, there is only a lengthening of the chain of the siloxane with the SiH functionalities.
In contrast, EP-A-0 786 488 describes a process for the preparation of siloxanes which consist of blocks which carry SiH functionalities and blocks which are free of functional groups. However, in these siloxanes, the SiH-containing blocks consist exclusively of siloxane groups in which an alkyl group and a hydrogen atom are bonded to each silicon atom.
The functionalization density of these blocks is accordingly extremely high. In the hydrosilylation, such high functionalization densities often tend to be disadvantageous, since it is extremely difficult to achieve a quantitative reaction of the SiH functionalities without secondary reactions.
As a result, SiH frequently remains in the end product, especially if a hydrosilylation with bulky olefins is carried out. Residues of SiH functionalities can undergo uncontrolled reactions during storage (for example elimination of hydrogen), which may lead to considerable problems.
As a source of the SiH groups, EP-0 786 488 uses D4H (1,2,3,4-tetramethylcyclotetrasiloxane), which, owing to its extremely high reactivity (spontaneous ignition), is a chemical which is difficult to handle on industrial scales.
In addition, the described process for the preparation of the siloxanes having a block structure does not offer the possibility of establishing the functionalization density of the SiH-containing block in a targeted and defined manner.